drm/{i915, xe}: move fbdev fb calls to parent interface

drm/{i915, xe}: move fbdev fb calls to parent interface

Move the driver specific fbdev fb calls to the display parent
interface. Reuse the existing struct intel_display_bo_interface, as this
is mostly about gem objects.

Put everything behind IS_ENABLED(CONFIG_DRM_FBDEV_EMULATION) to catch
configuration issues at build or link time.

v2: Rebase

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com> # v1
Link: https://patch.msgid.link/a6bb24909a58181cfc41b91a4c6538a181d27158.1774950508.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>